/Engine/Source/Runtime/Engine/Classes/Engine/LocalPlayer.h
This function will give you two points in Pixel Space that surround the World Space box.
False if there is no viewport, or if the box is behind the camera completely
bool GetPixelBoundingBox ( const FBox & ActorBox, FVector2D & OutLowerLeft, FVector2D & OutUpperRight, const FVector2f * OptionalAllotedSize )